1. Technical Field of the Invention
The present invention relates to a communication system which has a radio interface and a network interface, and particularly to a communication system that is suitably used for a case in which a comparatively large amount of data are acquired in the network interface by using a mobile telephone for carrying out a communication by using, for example, a code division multiple access system.
2. Description of the Prior Art
Various wireless terminals, such as mobile telephones and portable computers provided with wireless functions, have been generally used. These can transmit and receive data in the same manner as comparatively large desk top computers. Moreover, since these apparatuses can execute communications everywhere, these have come to be widely used on business, etc. Recently, the wireless terminals are used not only for electronic mails, but also for accessing and browsing home pages on the world wide web (WWW) and for downloading various kinds of data.
FIG. 6 schematically shows a communication system in which a mobile telephone is used for utilizing the Internet. Mobile telephones 1011 to 101N are wirelessly connected to a base station 102. The base station 102, which is shown as only one station for convenience of explanation, is connected to a mobile communication network 103 so that the mobile telephones 101 are allowed to communicate with each other through the base station 102 of the mobile communication network 103. The mobile communication network 103 is connected to the Internet 104. Apparatuses, such as contents servers 1051 to 105M for storing various contents of information and personal computers 106, etc., are connected to the Internet 104.
When a personal computer 106 in an office or home is used for accessing home pages so as to browse the page or download files, the connection is made to the Internet 104 usually through a cable network such as a public telephone network, thereby designating one of the uniform resource locators (URL) of the contents servers 1051 to 105M. to acquire contents that one wants.
When a mobile telephone 101 is used for accessing a home page, the same processes are basically carried out. However, there is a great difference in the way how the contents are downloaded from the contents servers 1051 to 105M.
The personal computer 106 can download the data in a comparatively short period of time, because it is connected through a cable network, although the time period depends upon transfer rates of modem and communication channel.
On the other hand, the transfer rate of the radio interface between the mobile telephones 1011 to 101N and the base station 102 is lower in general than that of the network interface from the base station 102 to the contents server 1051 to 105M, because the radio interface is designed for telephone conversation and transfer of small size data. Therefore, the transfer rate for a file with relatively large size is regulated by that of the radio interface. In Japanese Patent Application No. 10-529474 (1998), the transfer rate is not fixed by taking this special point into consideration.
In order to explain this problem concretely, a relationship between the transmission power and the distance from the base station to the mobile telephones is explained, referring to FIG. 7. A broken line indicates the border of a cell 111 which the base station 102 controls. The first mobile telephone 1011 is located in the vicinity of the border of the cell 111. The second mobile telephone 1012 is located at the middle point between the border of the cell 111 and the base station 102. The third mobile telephone 1013 is located in the vicinity of the base station 102. The first to third mobile telephones 1011 to 1013 are connected with the base station 102 by the code division multiple access (CDMA).
In this manner, the mobile telephones 1011 to 1013 have respectively different distances from the base station 102.
Further, if it is assumed that the mobile telephones 1011 to 1013 communicate with the base station 102 by transmitting electromagnetic waves having an identical intensity, the base station 102 receives electromagnetic waves having different signal levels from the three mobile telephones. As a result, a signal received from the first mobile telephone 1011 located at the farthest position from the base station 102 is influenced by that from the third mobile telephone 1013 located at the nearest position from the base station 102. Therefore, the base station 102 can not receives the signals properly. The time division multiple access system (TDMA) and the frequency division multiple access system (FDMA) are less susceptible to such problems, because the timing of transmission and reception is divided for users in TDMA and the signal frequencies are divided for users in FDMA, although this is not always true for the TDMA which employs a single frequency bandwidth.
Therefore, conventionally, the base station 102 checks the signal levels received from the first to third mobile telephones 1011 to 1013, and controls the transmission powers outputted from the mobile telephones 1011 to 1013 so as to increase or decrease them so that these signal levels are almost equal to each other on the base station 102 side. Concretely, in this example, the transmission power from the first mobile telephone 1011 is increased, while the transmission power from the third mobile telephone 1013 is decreased, on the premise that the data transfer rate at which the respective mobile telephones 1011 to 1013 transmit data to the base station 102 is of a single value. The premise is based on the fact that the transmission powers can not be of a single value, if the data transfer rates are of plural values.
The data transfer rate at which the base station 102 transfers data to the respective mobile telephones 1011 to 1013 is also set to be of a single value. The transfer rate is fixed to be of a single value for transferring voices and small files.
FIG. 8 illustrates a relationship between the network interface and radio interface in their data transfer amount in this conventional communication system. Since the network interface 121 is employed between the contents server 105 and the base station 102, the network interface has a comparatively large capacity channel (high-speed communication channel) 122 so as to transfer a large amount data. Since the radio interface 123 used mainly for telephone conversation is employed between the base station 102 and the portable telephone 101, the radio interface has a comparatively small capacity channel (low-speed communication channel) 124. Therefore, in an attempt to download a file, etc., having a comparatively large capacity from the contents server 105, the communication time period becomes long, due to the small capacity channel 124. In other words, in an attempt to access a home page by utilizing the Internet 104 (FIG. 6), it spends much time to transfer a large file, resulting in a slow response and a failure to carry out a smooth operation.